Must I wait for the one to whom my heart is attached, or leave the matter? And can one perform Istikhara prayer to remove confusion between two matters? What is the preferred supplication for Istikhara, and can two people be combined in Istikhara?

Istikhara is not for knowing the unseen, but rather for the servant to entrust his Lord with choosing what is best for him in his religious and worldly affairs when contemplating a permissible matter. Istikhara can be performed for multiple matters by the servant praying two rak'ahs, then supplicating with the Istikhara supplication for each matter separately. It is advisable to ascertain the desire of your friend's brother's brother for marriage. If he is not interested, then disregard him and accept whoever proposes to you if you are pleased with his religious commitment and character. If he is interested, then let him propose. If you hesitate between him and another suitor, then prioritize the one who is better in religious commitment and character.